Generally, most buildings should have sufficient bearing capacities so that the foundation ground thereof supports these buildings. If not sufficient, subsidence occurs in the uppermost portion or the deep-seated portion of the foundation ground, resulting in deterioration of stability of a building that is built in an upper portion of the foundation ground.
Therefore, it should be necessary to investigate, through suitable various inspections such as geological investigation and soil exploration, whether a bearing capacity of the ground can sufficiently withstand the weight and load of a building applied to the ground. In the ground such as a reclaimed land, the unconsolidated ground, the ground decomposing organic substance layers, a peatland, a wetland, the ground having significant change in moisture, or the ground having lots of voids or being ununiform, a bearing capacity of the foundation ground is not sufficient, and thus higher bearing capacity is required for the foundation ground.
Also, in order to strengthen a foundation for a structure on the ground, a plurality of piles is driven into the soft ground, or the foundation is made with reinforced concrete by digging widely and deeply, and then the structure is constructed on this foundation. When various structures and facilities are present around the construction site, a condition for strengthening the foundation is often not formed. Also, if the foundation is explored widely while not exactly knowing positions of underground utilities, damage of utilities such as gas pipes is also caused.
Therefore, as a method for securing a bearing capacity about the foundation ground considering the foregoing matters, using a pile foundation reinforcing method has been well known. In addition to the above, there have been suggested various construction methods involving a grouting construction method in which a drilling operation is performed on the foundation ground using a hydraulic drill and a rod and bit of various drilling machines, a steel pipe such as rebar is inserted into the drilled hole, and then a reinforcing solution (a grouting liquid) is injected. Among these construction methods, a micropile can be considered a representative example.
This micropile started in Italy in 1950s, and then has been constructed globally for the purpose of reinforcing the ground and replacing a pile. The micropile has been called a mini pile, a micro pile, a root pile, and a GEWI pile, or the like depending on application purpose and range for each country.
A construction method of a conventional micropile is mainly divided into a drilling step, a steel bar inserting and installing step, a grouting step, and a head part finishing step.
First, a drilled hole is formed using bits having various diameters such as 76 mm, 80 mm, 90 mm, 105 mm, 115 mm, 152 mm, and 165 mm, and in a special case, bits having diameters of 200 mm or more would be used. Also, in the unstable ground, a casing is installed to a depth at which an inner wall of the drilled hole does not collapse, and then the inside thereof is drilled by using a bit to form a drilled hole.
When a drilling operation is completed, a steel bar combined with one rebar, or three or more rebar is inserted and installed.
When the steel bar is inserted and installed into the drilled hole, a grouting material is injected. That is, gravity grouting is performed right after a pile body is installed in the drilled hole. Here, the grouting is repeated about 3-6 times so as to compensate contraction phenomenon of the grout material.
When the grouting is completed, the head part finishing step is performed, in which a steel plate is fastened with a nut, or welding is carried out at an upper portion.
However, according to a conventional micropile construction method, this method is only possible for the foundation ground having bedrock, and when a micropile is constructed in the ground where only soil layers are present, it is impossible to secure high bearing capacity.
Also, since the steel bar constituting the micropile has a smaller diameter with respect to the length thereof, an end area of the pile is much smaller than an embedded vicinity area, and thus there has been a limitation that an end bearing capacity of the micropile is not generally considered in design.
In addition, during the grouting, the grouting material starts to fill the bottom portion of the drilled hole and is then injected until the grouting material flows out of the entrance of the drilled hole. Since a cementation time is long, and the grouting is repeated about 3-6 times so as to compensate the contraction phenomenon, constructability deteriorates, a construction period is getting longer, and an injection pressure cannot be uniformly maintained. Therefore, it is difficult to check a state filled with the grouting material and it is not easy to manage quality.